A person who thinks they can work out the motives, actions and guilt of people involved in high profile media criminal cases, based only on media revelations and their limited imagination.

Derivation is from Lt Columbo (a fictional TV Homicide detective from the 70's and 80's) who always knew who the killer was, within 2 minutes of meeting them (although, because this was fiction, he was always right)

Armchair Columbo's exist in a simple world full of Black and white motives, stranger dangers and Paedophiles hiding behind every tree

They are more likely to be conversant with TV police dramas than real life, and will know a great deal about forensic science from such shows as CSI:NY and NCIS, without realizing that most of this is fiction

Tigerlily: On the news, it said the missing boy was fully dressed and walked out the front door. How could a 3 year old dress himself and open the front door? I reckon the parents must know a lot more than they are letting on
Vinnie: Oh Tigerlily, you're such an Armchair Columbo